US Secretary of State John Kerry said on Sunday major differences remain between Iran and six world powers in talks over the country’s controversial nuclear programme, an opinion shared by Tehran just a week before a July 20 deadline to reach a deal.

The United States, Britain, France, Germany, Russia and China want Iran to reduce its nuclear fuel-making capacity to deny it any means of quickly producing atom bombs. In exchange, international sanctions that have crippled the large OPEC member’s oil-dependent economy would gradually be lifted.

Iran, however, has said it is enriching uranium for peaceful energy purposes only, and wants the sanctions removed swiftly. But a history of hiding sensitive nuclear work from UN inspectors has raised international suspicions, as well as fears of a new Middle East war if diplomacy fails to yield a long-term agreement.

“Obviously we have some very significant gaps still, so we need to see if we can make some progress,” Kerry said ahead of meetings with European Union foreign policy chief Catherine Ashton and other EU foreign ministers who flew into the Austrian capital Vienna at the weekend to breathe new life into the negotiations.

Iranian Deputy Foreign Minister Abbas Araqchi echoed Kerry’s remarks. He was quoted by Iran’s Arabic language al-Alam television as saying that “disputes over all major and important issues still remain. We have not been able to narrow the gaps on major issues and it is not clear whether we can do it.”

Kerry arrived in Vienna early on Sunday, after brokering a deal in the Afghan capital Kabul the day before to end the country’s election crisis.

“It is vital to make certain that Iran is not going to develop a nuclear weapon and that their programme is peaceful, and that’s what we’re here to try and achieve and I hope we can make some progress,” Kerry said in Vienna.

Neither pessimistic nor optimistic

Araqchi said that he was “not pessimistic but also not very optimistic” about the chances for an agreement with the sextet ahead of the self-imposed deadline. “No proposal has been accepted yet. We have not reached any agreement over the enrichment (programme of Iran) and its capacity.”

He added that if the talks collapsed, Iran would resume higher-level enrichment that it suspended on January 20, when a preliminary deal that had been struck by all sides two months before took effect. Iran won limited relief from sanctions in return.

The November 24 deal included a provision for lengthening talks on a permanent agreement by up to six months if all sides agree. Araqchi said “there is a possibility of extending the talks for a few days or a few weeks if progress is made.”

A senior US official said on Saturday that an extension would be difficult to consider without first seeing “significant progress on key issues”.

French Foreign Minister Laurent Fabius also raised the possibility of extending the negotiations.

“If we can reach a deal by July 20, bravo, if it’s serious,” he told reporters. “If we can’t, there are two possibilities. One, we either extend ... or we will have to say that unfortunately there is no prospect for a deal.”

British Foreign Secretary William Hague said it was too early to speak of extending the diplomacy. “It is unlikely there will be a quick breakthrough today but we...shall see what scope there is for making progress before July 20,” he told reporters.

Failure to seal a deal would mean the limited sanctions relief currently in place for Iran would end and Tehran could expect tougher sanctions, above all from the United States.

The Russian and Chinese foreign minister were not in Vienna on Sunday due to a meeting in Brazil of the BRICS developing countries - Brazil, Russia, India, China and South Africa. Moscow and Beijing sent senior diplomats to Vienna instead.